This invention is in the field of disk drive control circuits, and is more specifically directed to a preamplifier for a read/write head in a hard disk drive.
Continuing progress toward higher performance yet less expensive personal computers, including both desktop workstations and portable computers, has resulted in large part from advances in nonvolatile data storage technology. As is well known in the art, the capacity of conventional disk drives has greatly increased over recent years, at ever decreasing cost per megabit. This capacity increase is directly related to improvements in the density with which data can be stored in a magnetic disk drive, particularly in “hard” disk drives (i.e., disk drives in which the magnetic disk is not removable from the location of the read/write heads). Advances in disk drive technology have decreased the surface area required to reliably and retrievably store a bit along a “track” on the disk surface, and have also decreased the spacing between adjacent tracks. This reduction in the active disk surface area per unit of storage has been enabled, in large part, by corresponding reductions in the size and precision of the magnetic transducers, commonly referred to as “heads”, that effect the writing and reading operations in magnetic disk drives.
In conventional magnetic disk drives, the writing and reading of stored data is carried out by way of near-field magnetic processes. To write data, ferromagnetic domains at the disk surface are selectively oriented by applying a magnetic field in close proximity to the disk surface. One type of conventional write head is the well-known inductive writer, which includes an electromagnet having a gap that can be positioned near the magnetic disk surface. The electromagnet is selectively energized to establish a magnetic field, at the gap, that is strong enough to define a magnetic “transition pattern” of the desired polarity at the addressed location of the disk surface. Data is read from the disk by sensing the polarity of the magnetic field established by these magnetic transition patterns. Conventional read heads include inductive heads consisting of an electromagnet (which may be the same electromagnet used to write data) in which a current is induced by the magnetic fields at the disk surface; more recently, read heads are implemented by a magnetoresistive (MR) head having a resistance that varies with the polarity of the magnetic field.
In modern disk drives, the read/write heads are disposed within a “slider” at the distal end of a head gimbal assembly (HGA) suspension. The flexible HGA suspension is attached to an actuator, which includes a so-called “voice coil” motor that positions the heads at the desired locations of the disk surface. The relative motion between the spinning disk surface and the slider creates a lifting force on the slider, establishing an air bearing surface (ABS) on which the slider rides over the disk surface. Typically, the heads are located at the trailing edge of the slider, which is typically closer to the disk surface than is the slider leading edge.
In connection with these near-field mechanisms, the magnetic field strength varies exponentially as the distance between the magnetic transducers (read/write heads) and the magnetized disk surface shrinks. It has been observed that the areal density of data storage at the disk surface, for a given bit error rate (BER), depends strongly on the distance between the heads and the disk surface. The spacing maintained by the air bearing surface (ABS) between the read/write heads and the disk surface is referred to in the art as the “fly height” of the heads. In modem conventional disk drives, the mean fly height is on the order of a few nanometers. Low fly heights have enabled the very high areal densities attained in modem disk drives.
However, low fly heights tend to increase the wear of both the disk surface and the read-write heads. At extremely low fly heights, relatively small asperities in the disk surface can cause contact between the slider and the disk surface, depleting and degrading lubricants, causing wear on both the slider and the disk surface and causing contamination from wear particles, and in some cases causing the heads to stick at locations of the disk surface where contact is made.
Disk drive manufacturers are thus faced with a tradeoff between disk drive reliability, on one hand, and areal density and BER, on the other hand, in determining the desired fly height of the read/write heads. Accordingly, significant effort has been and is being expended in the art to provide extremely smooth magnetic disk surfaces, so that very low fly heights can be achieved while still providing reasonable disk durability. By way of further background, active control of the fly height by way of a micromechanical air valve is described in U.S. Pat. No. 6,578,816 B1.
It is also known that the writing current conducted by the inductive write head causes resistive heating at the read/write head, and in turn causes thermal expansion of the write head and, in some cases, also of the MR read head (or inductive read head, if present). This thermal expansion typically causes reduction in the fly height. But if the nominal fly height of the read/write head is already extremely small, this thermal expansion of the read/write head can cause contact between the disk surface and the read/write head, resulting in disk and head wear and in the degradation of the disk system described above.
By way of further background, it is known to include a resistor within the slider of a disk drive read/write head for controlling the fly height. An example of this construction is described in U.S. Pat. No. 5,991,113. As described in this reference, the application of a current to the resistor heats and thus expands the read/write heads, reducing the fly height of the read/write heads from the disk surface.